1. Field of the Invention
The present invention relates to an apparatus for driving an optical system for use in an optical recording and reproducing apparatus.
2. Related Art Statement
An apparatus for driving an optical system for use in an optical recording and reproducing apparatus generally comprises a focusing mechanism for moving an optical element, e.g. an objective lens in a focusing direction which is perpendicular to a plane of an optical record medium and a tracking mechanism for moving the optical element in a tracking direction which is perpendicular both to the focusing direction and to a direction of a track formed on the record medium. The focusing mechanism serves to compensate a vibration of the record medium in a direction perpendicular to the plane of the record medium. Since this vibration of the record medium is small, the optical element is usually supported movably by means of a spring mechanism. In the tracking mechanism, the optical element is moved to follow an information track on an optical record disc by moving the optical element in a radial direction, and thus the optical element is usually supported by a combination of shafts and bearings such as radial bearings and slide bearings.
FIG. 1 is a perspective view showing a known optical system driving apparatus. In FIG. 1, an objective lens (optical element) 91 is supported by a holder 92 and the holder is secured to a carriage 94 by a pair of leaf springs 93a and 93b movably in a focusing direction Z. The carriage 94 is supported by a pair of shafts 96a and 96b by means of bearings 95a and 95b, respectively movably in a tracking direction X. The shafts 96a and 96b are supported by a stationary member not shown.
In this known optical system driving apparatus, by applying a driving force to the carriage 94 in the tracking direction X, the objective lens 91 can be moved in the tracking direction X together with the carriage 94. By applying a driving force to the holder 92 in the focusing direction Z, the objective lens 91 can be moved in the focusing direction Z.
In the known optical system driving apparatus illustrated in FIG. 1, the movement of the objective lens 91 in the focusing direction Z is performed by directly applying the driving force to the holder 92 which holds the objective lens 91. However, the movement of the objective lens 91 in the tracking direction X is carried out indirectly by means of the leaf spring 93aand 93b, because the driving force is applied to the carriage 94 instead of to the holder 92. It should be noted that the leaf springs 93a and 93b are used to move the objective lens 91 in the focusing direction Z, and thus they should have weak stiffness in the focusing direction Z. Furthermore, the leaf springs 93a and 93b have rather weak stiffness in the tracking direction X.
Therefore, in the known optical system driving apparatus shown in FIG. 1, when the objective lens 91 is moved in the tracking direction X, an undesired resonance might occur due to the leaf springs 93a and 93b and an accurate tracking control could not be performed. If this undesired resonance is suppressed, a dynamic control frequency range might be narrowed. This apparently degrades the performance of the apparatus.
The above mentioned undesired resonance of the leaf springs 93a and 93b becomes large when carriage 94 is moved in the tracking direction X. However, even when the objective lens 91 is moved in the focusing direction Z, there might occur a resonance due to a deviation between a driving force applying point and a point of gravity of the holder 92 which is moved in the focusing direction Z. In this manner, not only the tracking control, but also the focusing control could not be performed accurately. It should be further noted that the above mentioned undesired phenomenon occurs not only in the apparatus shown in FIG. 1, but also in another known optical system driving apparatuses in which the holder holding the objective lens is supported by resilient members such as four wires or leaf springs movably in the focusing direction Z as well as in the tracking direction Z.
Recently the optical system driving apparatus has been required to be small in size and light in weight, so that stiffness of the holder 92 and carriage 94 is liable to be weak. Then, not only the springs 93a and 93b, but also the holder 92 and carriage 94 might be subjected to undesired resonance and the focusing control and tracking control could not be carried out accurately. For instance, when the holder 92 might be subjected to a resonance, not only the control in the tracking direction X but also the control in the focusing direction Z are influenced, and thus the performance of the driving apparatus is decreased to a large extent.
In Japanese Utility Model Application Publication Kokai Hei 6-7449, there is proposed an optical system driving apparatus, in which the undesired resonance is damped by providing a weight via a vibration absorbing member to a carriage. In this known apparatus, the vibration absorbing member is made of a material having a resilient damping property and viscose damping property, and therefore the vibration absorbing member serves as a dynamic damper and the resonance is effectively suppressed. However, in this known apparatus, only the resonance of the carriage to which the dynamic damper is secured can be suppressed, and thus if this measure is applied to the known apparatus shown in FIG. 1, the resonance of the leaf springs 93a, 93b and holder 92 could not be suppressed at all.
It should be noted that the dynamic damper may be applied to the holder 92. Then, the resonance of the holder 92 and objective lens 91 due to the resonance of the leaf springs 93a and 93b could be suppressed. However, in this case, a weight of the holder 92 is increased and the resonance frequency is decreased. Therefore, a wide control frequency range could not be attained.
In Japanese Patent Application Publication Kokai Hei 3-144924, there is shown another known optical system driving apparatus, in which a balancer is secured to an objective lens holder by means of an adhesive agent to take a balance with respect to the objective lens. In this apparatus, the balancer serves as a damper and the undesired resonance is suppressed.
However, if such a balancer is applied to the holder 92 of the apparatus shown in FIG. 1, a weight of the holder might be increased and a resonance frequency might be lowered. Therefore, the control frequency range might be narrow and the performance of the apparatus might be decreased. A part of the holder may be divided and a divided part is secured to the remaining holder by means of an adhesive agent. In this case, the divided part may serve as the damper. However, a weight of the divided portion is light, so that it could not serve as the damper effectively.
In Japanese Utility Model Application Publication Kokai Sho 60-48377, there is disclosed a voice coil motor for driving a carriage, in which the coil is secured to the carriage by means of damping member. In this apparatus, a resonance of the coil itself can be suppressed, but a resonance of the carriage could not be suppressed.